One of the major requirements for III-N material based high voltage power devices is that the device stack needs to be highly electrically resistive with a high leakage current blocking capability. This leakage current blocking capability is usually evaluated with the voltage at a vertical leakage current density of 1 μA/mm2 at both room temperature (25° C.) and a high temperature (typically 150° C.) when the structure is biased vertically with both polarities. Due to a lack of native substrates, the III-N material based devices are usually grown on foreign substrates, such as Si, sapphire, and SiC by using metal organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). This type of hetero-epitaxy process then requires growth of buffer layers which manage the lattice mismatch introduced stress so that the device active layers can be grown with satisfactory material quality and electric properties. In addition, the stress management buffer layers can in most cases also contribute to the leakage blocking capability of the full stack.
One common practice to enhance the stack leakage blocking capability is to simply increase the buffer thickness. A superlattice (SL) buffer is a type of buffer structure which usually consists of multiple repetitions/periods of AlN/AlxGa1-xN (0=<x<1) pair layers. For growth of a device stack on a Si substrate with a SL buffer, the SL buffer must be very thick (3-4 μm) to provide sufficient leakage blocking capability. Such thicknesses lead to difficulties in providing sufficient in situ convex wafer bow in order to compensate for thermal mismatch introduced concave wafer bowing during post-epitaxy cooling.
From US 2012/223328 A1, a group III-nitride epitaxial laminate substrate is known comprising a substrate, a buffer, and a main laminate in this order, wherein the buffer includes an initial growth layer, a first superlattice laminate, and a second superlattice laminate in this order. The first superlattice laminate includes five to 20 sets of first AlN layers and second GaN layers, the first AlN layers, and the second GaN layers being alternately stacked, and each one set of the first AlN layer and the second GaN layer has a thickness of less than 44 nm. The second superlattice laminate includes a plurality of sets of first layers made of an AlN material or an AlGaN material and second layers made of an AlGaN material having a different band gap from the first layers, the first and second layers being alternately stacked. The dual superlattice laminate structure is used to achieve both an improvement in crystallinity and a suppression of substrate warpage.